R-dependent ionization of the valence orbitals of I2 by strong laser fields

Using a pump-dump-probe technique and Fourier-transform spectroscopy, we study the internuclear separation $R$ dependence and relative strength of the ionization rates of the $\pi$ and $\sigma$ electrons of I$_2$, whose valence orbitals are $\sigma_g^2\pi_u^4\pi_g^4\sigma_u^0$. We find that ionization of the HOMO-2 ($\sigma_g$) has a strong dependence on $R$ while the HOMO and HOMO-1 do not. Surprisingly, the ionization rate of the HOMO-2 exceeds the combined ionization rate of the less bound orbitals and this branching ratio increases with $R$. Since our technique produces target molecules that are highly aligned with the laser polarization, the $\sigma$ orbitals will be preferentially ionized and undergo enhanced ionization at larger $R$ compared to the $\pi$ orbitals. Nevertheless, it is highly unusual that an inner orbital provides the dominant strong field ionization pathway in a small molecule.